A simple plan - cnidarians and the origins of developmental mechanisms

Key Points Cnidarians lack true mesoderm, in the sense of a third germ layer that arises as a direct result of gastrulation. However, at the molecular level, muscle development in the medusa of the hydrozoan Podocoryne carnea seems to mirror its differentiation as a mesodermal lineage in 'higher' animals. Molecular evidence indicates that the Anthozoa are the basal cnidarian group, but they lack both the medusa stage and well-developed muscle. So, there is active debate as to whether the molecular similarities between P. carnea muscle development and that in Bilateria indicate convergent evolution or the loss of the medusa stage in the Anthozoa. Many of the same 'mesodermal marker' genes are expressed during gastrulation in Anthozoa and Bilateria. This might indicate that the primitive function of these genes was in such fundamental processes as cell proliferation, adhesion and motility, and indicates the possibility that as true mesoderm evolved from endomesoderm, these genes became associated with the former. During early coral and anemone development, a second axis is defined by the expression of a gene related to DPP/BMP4 — the key determinant of the dorsal–ventral axis in higher animals — and this asymmetrical expression is maintained in the sea anemone polyp. Molecular analyses establish the bilateral anthozoans, rather than the radial hydrozoans, as the basal cnidarians. Anthozoans have several of the regulatory genes that are responsible for patterning both of the principal axes of higher animals (for example, Emx and dpp / Bmp4 ), and express them in ways that are inconsistent with the assumption of a single body axis. EST projects on several cnidarians consistently highlight the complexity of the cnidarian gene complement and imply that, at the genetic level, the common ancestor of bilateral animals was more complex than has previously been assumed. The blurring of the triploblast/diploblast divide, the confirmation that basal cnidarians are bilateral and the complexity of their gene set lead us to suggest rethinking the entire notion of cnidarians as 'lower' animals. Several key developmental genes have been independently duplicated in cnidarians, including snail and Hox -related genes, and, even at the genus level, large changes in genome size have occurred. These factors complicate the understanding of the evolution of specific gene families and will need to be considered in selecting representative cnidarians for genomic sequencing. Cnidarian